The present invention generally relates to methods for the practical implementation of nanocrystalline or amorphous metals or alloys as coating materials. More particularly, methods of applying such nanocrystalline or amorphous metals or alloys to high volume electrodeposition operations, to continuous electrodeposition operations, and to the rebuilding and reworking of components are presented.
Industrial applications, such as high-volume electrodeposition production, barrel plating, continuous electrodeposition, and rework/rebuild require coating materials with specific properties. There is a continual need for new and improved coating materials for these applications, which can offer economic benefits or improved product properties.
High Volume Electrodeposition:
High volume electrodeposition coating processes, such as barrel plating, are economically and practically desirable for coating many components simultaneously. However, insufficient coating properties create significant challenges for these high volume electrodeposition coating processes.
High volume electrodeposition processes such as barrel plating generally involve more than two components being plated simultaneously, and which components may be in electrical contact with one another during at least part of the process. The parts may also experience contact mechanical loads and/or abrasive loading at the electrical contact points. Such loading may be increased if the components experience agitation during the process.
In design of high volume electrodeposition processes, an important issue is the character and properties of the deposited coating. In general, a weak or poorly adhered coating may be damaged by the agitation process, as components shift their relative positions and give rise to sliding contact points or local impacts on the component surfaces. Similarly, soft and malleable coatings, or those with low hardness, low resistance to wear, indentation, or frictional sliding damage, may acquire defects such as cracks, scratches or delaminations during the process. It is therefore important that the deposited coating have desirable properties that resist damage during processing, and that the process characteristics be controlled to avoid such damage.
Another coating property of importance to the efficiency and efficacy of a high volume electrodeposition process is its electrical conductivity. Because the electrical connection of each component to the power supply is achieved, in general, through contacts between components or between components and the electrical lead connected to the power supply, electrical current is required to pass across the surfaces of the components. As the deposition process proceeds and the components become coated, electrical current is required to pass through the coating material itself. If the coating is of low electrical conductivity, current flow is discouraged, reducing the efficiency of the deposition. For this reason, coatings of relatively higher electrical conductivity are generally more appropriate to high volume electrodeposition processes such as barrel plating.
An example relating to the electrical conductivity of electrodeposited coatings is provided by the case of hexavalent chromium deposits. Coatings of chromium produced by deposition from the hexavalent bath are desirable in many respects, due to the high hardness, wear resistance, and corrosion resistance of the coating. However, the electrical conductivity of hexavalent chromium coatings is low compared to many metals, and reduces the efficiency of a high volume process such as barrel plating. This renders such operations economically difficult to sustain.
A need has long existed for new electrodeposited coatings which combine new suites of properties, to be produced in high volume with such techniques. For example, it would be desirable to use a high-strength, strong adhesion, abrasion resistant nanocrystalline or amorphous coating with high electrical conductivity, to improve both the quality of the coating and coated product, as well as increase the efficiency of the process. Additionally desired properties include higher hardness, ductility, wear resistance, electrical properties, magnetic properties, corrosion characteristics, substrate protection, improved environmental impact, improved worker safety, improved cost, and many others.
Continuous Electrodeposition:
Continuous electrodeposition processes are economically and practically desirable for applying a coating onto a strip of material. A need has long existed for coatings being applied using continuous electrodeposition which create a final product with more desirable properties. For example, higher hardness, strength, ductility, wear resistance, electrical properties, magnetic properties, corrosion characteristics, substrate protection, improved environmental impact, improved worker safety, improved cost, and many others.
Rework/Rebuild:
Rework/rebuild processes are economically and practically desirable for correcting deficiencies in products. A critical step in the rework/rebuild process is the application of a suitable coating material. One common material used for this coating process is hard electrodeposited chromium, alternatively called “hard chromium” or “hard chrome”. Rework/rebuild is a common procedure for chromium plating facilities, in which hard chromium is the material plated as a coating. Frequently, the chromium coating will be up to or in excess of 375 μm in thickness prior to the machining step. K. O. Legg cites rework and rebuild operations as comprising one of the largest single uses of hard chromium plating in his article “Overview of Chromium and Cadmium Alternative Technologies” (in Surface Modification Technologies XV, edited by T. S. Sudarshan and M. Jeandin, ASM International, Materials Park OH, 2002), which is fully incorporated herein by reference. A drawback of hard chromium coatings for rework/rebuild operations is the toxicity and carcinogenicity of the chemicals used in the coating process; these have serious implications for the environment and for worker safety.
Other coating technologies can be applied to rework operations, including but not limited to other electroplated metal technologies, electroless coatings, plasma or thermal spray coatings, and physical vapor deposition coatings. These coating technologies are generally more expensive than is hard chromium coating, but can mitigate the negative environmental issues associated with hard chromium. The main requirements for the coating used in rework/rebuild operations are that it be deposited to sufficient thickness, that it have the desired surface properties (i.e., resistance to corrosion, abrasion, erosion, wear, fatigue, etc.), that it adhere to the base material of the substrate component, and that it can be machined by a suitable method to exhibit the correct geometry.
Other factors may influence the choice of a coating technology for use in rework/rebuild operations. For example, the geometry of the component may preclude some coating technologies. Plasma spray coatings are not generally useful for coating internal diameters of bores or other re-entrant geometries, and so could not be used for rework/rebuild except for regions of the component material that may be connected by a line-of-sight to the spray nozzle. Similarly, hard chromium plating is often said to be a “low throwing-power” process, meaning that the process preferentially deposits chromium on portions of the component closer to a line-of-sight with a nearby plating anode. Many anodes are often used in parallel to improve the density of “sight lines” to the component and provide a uniform coating, but the coating of recesses, internal surfaces, and re-entrant geometries is often non-uniform. For these reasons, rework/rebuild operations on complex surfaces are generally more challenging than those on simpler geometries.
Accordingly, a need has long existed for coatings, coating materials, and coating application processes to be used in rework/rebuild operations that would provide the following: high strength and hardness, high corrosion resistance, high wear and abrasion resistance, thicknesses of at least 200 μm, improved environmental impact, improved worker safety, improved cost, improved ability to coat geometries with internal surfaces and non-line-of-sight surfaces, better compatibility or matching of the substrate material to the rework/rebuild coating, improved surface properties, the ability to withstand subsequent machining operations, and the ability to utilize existing electroplating equipment.